1. Field of the Invention
The invention relates to paper reprocessing, and more particularly to the repulping of wet strength broke.
2. Background
Broke is waste or off-spec paper which is to be recycled. It is generally more desirable to recycle the broke by a process called repulping, than it is to merely dispose of the broke as waste paper.
In the repulping process, the cellulose fibers which make up the broke are sufficiently separated from the broke to make them usable for manufacturing paper. Repulping wet strength broke is difficult because such broke contains a wet strength resin (such as a polyamide-epichlorhydrin resin) added during paper production to enhance the strength of the paper produced so that the paper does not fall apart when used under wet conditions. The wet strength resin binds the cellulose fibers together, forming a water impervious coating, which impedes the repulping process goal of separating the cellulose fibers. Paper towels, tissues, food wrappings, and other paper products are typically treated with wet strength resins to prevent their deterioration when used under wet conditions. Typically, paper treated with wet strength resins will retain at least 15% of the paper's dry strength when wet. Paper without wet strength resin generally retains only 2-7% of its dry strength when wet.
Oxidation facilitates the break down of the wet strength resin to permit separation of the cellulose fibers. Traditionally, hypochlorite, particularly sodium hypochlorite, has been used by paper mills in the repulping of wet strength broke to oxidize the wet strength resin to facilitate fiber separation. When so used, hypochlorite oxidizes the wet strength resins within a narrow, carefully maintained pH range and within a temperature range of from about 122.degree. F. (50.degree. C.) to 150.8.degree. F. (66.degree. C.). After broke has been successfully repulped, an antichlor is added to neutralize the remaining chlorine.
Environmental issues have been raised concerning the use of hypochlorite for repulping. These concerns relate to the formation of organic halides which are adsorbed by the pulp, chloroform emission, and the problem of adding toxic chlorinated hydrocarbons to the effluent stream. For these reasons, non-halogen containing compounds, such as persulfates have been used to oxidize wet strength resin during the repulping process.